(1) Field of the Invention
The present invention relates to transgenic plants which after harvest degrade the lignin and cellulose therein to fermentable sugars which can further be fermented to ethanol or other products. In particular, the transgenic plants comprise ligninase and cellulase genes from microbes operably linked to a DNA encoding a signal peptide which targets the fusion polypeptide produced therefrom to an organelle of the plant, in particular the chloroplasts. When the transgenic plants are harvested, the plants are ground to release the ligninase and cellulase which then degrade the lignin and cellulose of the transgenic plants to produce the fermentable sugars.
(2) Description of Related Art
If human economies are to become more sustainable, then it is imperative that humans learn how to use the solar energy that is carbon-fixed in plant biomass to meet a larger fraction of energy and raw material needs. About 180 billion tons of new plant matter (biomass) is produced annually worldwide. Thus, about 30 tons of plant matter per person is produced every year. In North America, about three tons of plant matter is used per person every year. That means that the energy value of naturally produced biomass is equivalent to ten times the total human use of all types of energy. However, because of the difficulty in extracting the energy from plant biomass, most of the energy potential of the biomass goes unused.
At present, the United States produces ethanol from starch produced in corn grain using amylase enzymes to degrade the starch to fermentable sugars. Much of the ethanol that is produced from corn grain is exported to Brazil where it is efficiently used to power transportation vehicles. In general, while the corn grain is used in the production of ethanol, the remainder of the corn biomass, i.e., the leaves and stalks, is seldom unused because of the cost in degrading the leaves and stalks comprising lignins and cellulose, generally in the form of lignocellulose, to fermentable sugars. The lignocellulose in the stalks and leaves of corn biomass represents a tremendous source of untapped energy that goes unused because of the difficulty and cost of converting it to fermentable sugars.
Currently, there are four technologies available to convert cellulose to fermentable sugars. These are concentrated acid hydrolysis, dilute acid hydrolysis, biomass gasification and fermentation, and enzymatic hydrolysis.
Concentrated acid hydrolysis is based on concentrated acid de-crystallization of cellulose followed by dilute acid hydrolysis to sugars at near theoretical yields. Separation of acid from sugars, acid recovery, and acid re-concentration are critical unit operations. The concentrated sulfuric acid process has been commercialized in the past, particularly in the former Soviet Union, Germany, and Japan. However, these processes were only successful during times of national crisis, when economic competitiveness of ethanol production could be ignored.
Dilute acid hydrolysis occurs in two stages to maximize sugar yields from the hemicellulose and cellulose fractions of biomass. The first stage is operated under milder conditions to hydrolyze hemicellulose, while the second stage is optimized to hydrolyze the more resistant cellulose fraction. Liquid hydrolyzates are recovered from each stage, neutralized, and fermented to ethanol. As indicated earlier, Germany, Japan, and Russia have operated dilute acid hydrolysis percolation plants off and on over the past 50 years. However, the technology remains non-competitive for the conversion of cellulose to fermentable sugars for production of ethanol.
In biomass gasification and fermentation, biomass is converted to a synthesis gas, which consists primarily of carbon monoxide, carbon dioxide, and hydrogen) via a high temperature gasification process. Anaerobic bacteria are then used to convert the synthesis gas into ethanol.
In early processes embracing enzymatic hydrolysis of biomass to ethanol, the acid hydrolysis step was replaced with an enzyme hydrolysis step. This process scheme was often referred to as separate hydrolysis and fermentation (SHF) (Wilke et al., Biotechnol. Bioengin. 6: 155–175 (1976)). In SHF, pretreatment of the biomass is required to make the cellulose more accessible to the enzymes. Many pretreatment options have been considered, including both thermal and chemical steps. The most important process improvement made for the enzymatic hydrolysis of biomass was the introduction of simultaneous saccharification and fermentation (SSF) U.S. Pat. No. 3,990,944 to Gauss et al. and U.S. Pat. No. 3,990,945 to Huff et al.). This process scheme reduced the number of reactors involved by eliminating the separate hydrolysis reactor and, more importantly, avoiding the problem of product inhibition associated with enzymes.
In the presence of glucose, β-glucosidase stops hydrolyzing cellobiose. The build up of cellobiose, in turn, shuts down cellulose degradation. In the SSF process scheme, cellulase enzyme and fermenting microbes are combined. As sugars are produced by the enzymes, the fermentative organisms convert them to ethanol. The SSF process has, more recently, been improved to include the co-fermentation of multiple sugar substrates in a process known as simultaneous saccharification and co-fermentation (SSCF) (www.ott.doe.gov/biofuels/enzymatic.html).
While cellulase enzymes are already commercially available for a variety of applications. Most of these applications do not involve extensive hydrolysis of cellulose. For example, the textile industry applications for cellulases require less than 1% hydrolysis. Ethanol production, by contrast, requires nearly complete hydrolysis. In addition, most of the commercial applications for cellulase enzymes represent higher value markets than the fuel market. For these reasons, enzymatic hydrolysis of biomass to ethanol remains non-competitive.
However, while the above processes have focused on converting cellulose to fermentable sugars or other products, much of the cellulose in plant biomass is in the form of lignocellulose. Lignin is a complex macromolecule consisting of aromatic units with several types of inter-unit linkages. In the plant, the lignin physically protects the cellulose polysaccharides in complexes called lignocellulose. To degrade the cellulose in the lignocellulose complexes, the lignin must first be degraded. While lignin can be removed in chemi-mechanical processes that free the cellulose for subsequent conversion to fermentable sugars, the chemi-mechanical processes are inefficient. Ligninase and cellulase enzymes, which are produced by various microorganisms, have been used to convert the lignins and cellulose, respectively, in plant biomass to fermentable sugars. However, the cost for these enzymes is expensive, about six dollars a pound. As long as the cost to degrade plant biomass remains expensive, the energy locked up in the plant biomass will largely remain unused.
An attractive means for reducing the cost of degrading plant biomass is to make transgenic plants that contain cellulases. For example, WO 98/11235 to Lebel et al. discloses transgenic plants that express cellulases in the chloroplasts of the transgenic plants or transgenic plants wherein the cellulases are targeted to the chloroplasts. Preferably, the cellulases are operably linked to a chemically-inducible promoter to restrict expression of the cellulase to an appropriate time. However, because a substantial portion of the cellulose in plants is in the form of lignocellulose, extracts from the transgenic plants are inefficient at degrading the cellulose in the lignocellulose.
U.S. Pat. No. 5,981,835 to Austin-Phillips et al. discloses transgenic tobacco and alfalfa which express the cellulases E2, or E3 from Thermomononospora fusca. The genes encoding the E2 or E3, which were modified to remove their leader sequence, were placed under the control of a constitutive promoter and stably integrated into the plant genome. Because the leader sequence had been removed, the E2 or E3 product preferentially accumulated in the cytoplasm of the transgenic plants. However, because the cellulase can leak out of the cytoplasm and into the cell wall where it can degrade cellulose in the cell wall, the growth of the transgenic plants can be impaired.
U.S. Pat. No. 6,013,860 to Himmel et al. discloses transgenic plants which express the cellulase E1 from Acidothermus cellulolyticus. The gene encoding E1, which was modified to remove the leader region, was placed under the control of a plastid specific promoter and preferably integrated into the plastid genome. Because the leader sequence had been removed, the E1 product accumulated in the plastid.
While the above transgenic plants are an improvement, accumulation of cellulytic enzymes in the cytoplasm of a plant is undesirable since there is the risk that the cellulase can leak out from the cytoplasm and injure the plant. For example, research has shown that plants such as the avocado, bean, pepper, peach, poplar, and orange also contain cellulase genes, which are activated by ethylene during ripening and leaf and fruit abscission. Therefore, transgenic plants which contain large quantities of cellulase in the cytoplasm are particularly prone to damage. Furthermore, the cellulases accumulate in all tissues of the plant which can be undesirable. Restriction of cellulase expression to plastids is desirable because it reduces the risk of plant damage due the cellulases leaking from the cell. However, for most crop plants, it has been difficult to develop a satisfactory method for introducing heterologous genes into the genome of plastids. Furthermore, cellulase is expressed in all tissues which contain plastids which can be undesirable.
For production of ligninases to use in degrading lignins, the ligninases of choice are from the white-rot fungus Phanerochaete chrysosporium. One of the major lignin-degrading, extracellular enzymes produced by P. chrysosporium is lignin peroxidase (LIP). Potential applications of LIP include not only lignin degradation but also biopulping of wood and biodegradation of toxic environmental pollutants. To produce large quantities of LIP, the fungus can be grown in large reactors and the enzyme isolated from the extracellular fluids. However, the yields have been low and the process has not been cost-effective. Production of recombinant LIP in E. coli, in the fungus Trichoderma reesei, and baculovirus have been largely unsuccessful. Heterologous expression of lignin-degrading manganese peroxidase in alfalfa plants has been reported; however, the transgenic plants had reduced growth and expression of the enzyme was poor (Austin et al., Euphytica 85: 381–393 (1995)).
Although difficult to sufficiently and cheaply produce ligninases in non-plant systems, ligninases have evoked worldwide interest because of their potential in degrading a variety of toxic xenobiotic compounds such as PCBs and benzo(a)pyrenes in the environment (Yadav et al., Appl. Environ. Microbiol. 61: 2560–2565 (1995); Reddy, Curr. Opin. Biotechnol. 6: 320–328 (1995); Yadav et al., Appl. Environ. Microbiol. 61: 677–680 (1994)).
Therefore, a need remains for an economical method for making transgenic crop plants wherein the ligninase and cellulase genes are incorporated into the plant genome but wherein the ligninase and cellulase expression are restricted to particular plant tissues, e.g., the leaves, and the ligninase and cellulase products are directed to a plant organelle wherein it accumulates without damaging the transgenic plant.